Signalling delegation in a moving network

ABSTRACT

In order to delegate location update signaling responsibility from a Mobile Node to a Mobile Router, the Mobile Router is provided with a second symmetric key generated by a Mobile Node using a first symmetric key shared between the Mobile Node and a Peer Node. The Mobile Router is additionally provided with a “certificate” authenticating the second symmetric key using the first symmetric key. In this way, the mobile router can sign location update related messages sent to the Peer Node with the second symmetric key, and can provide the Peer Node with the certificate in order to allow the Peer Node to authenticate the right of the Mobile Router to act on behalf of the Mobile Node.

TECHNICAL FIELD

The present invention relates to signalling delegation in a moving network and in particular, though not necessarily, to delegation of location update rights from a Mobile Node to a Mobile Router.

BACKGROUND

Trains, busses, airplanes and Personal Area Networks (PANs) are examples of use cases where moving network technologies can be applied. A moving wireless network is a cluster consisting of Mobile Nodes (MNs) and Mobile Routers (MRs). A Mobile Router routes IP traffic between a Mobile Node and the Internet (or other IP network). The Mobile Router acts as an access point for the Mobile Node. A Mobile Router may be connected to another moving network or directly to the Internet via an access point (AP). In the former case, a set of nested mobile networks results. Figure lA illustrates a scenario where a Mobile Node is initially attached to a Mobile Router. In FIG. 1B, the Mobile Node is handed-off to a second Mobile Router, whilst in FIG. 1C the Mobile Node is handed-off from the second Mobile Router to a fixed access router.

A Mobile Router broadcasts beacons to advertise its existence and that it provides a mobile routing service. The beacons may contain an operator identifier, for example, to aid selection of an appropriate network by a user. When a Mobile Node finds a suitable Mobile Router, the Mobile Node triggers an attachment exchange with the Mobile Router.

It will be appreciated that when a Mobile Router changes its point of attachment to the Internet, or is handed-off to another Mobile Router, the locations (i.e. IP addresses) of all of the downstream Mobile Nodes will change. In order to maintain service continuity during and after hand-off of a Mobile Router, some mechanism is required in order to ensure that data sent from peer (or correspondent) nodes involved in a communication session with the Mobile Node is sent to the Mobile Node at the node's new location. A number of approaches are possible.

Firstly, the Mobile Node itself can be responsible for notifying the Peer Node(s) of its new location. This approach results however in a large volume of location update related signalling for even relatively small moving networks, and results in relatively long hand-off reaction times. A second approach involves the creation of a tunnel between the Mobile Router and some home router (or “home agent”) within the fixed network. All traffic sent from a Peer Node to a Mobile Node (and possibly traffic sent in the reverse direction) is routed through the tunnel. When the Mobile Router changes its location, it sends an update to the home router. Two problems with this approach are the use of sub-optimal (triangular) routing and the increased packet size due to the tunnelling overhead. An example of this second approach is the IETF Network Mobility (NEMO) mechanism. A third approach involves the delegation of signalling rights from the Mobile Nodes to the Mobile Router to allow the Mobile Router to send location update signalling to Peer Nodes on behalf of the Mobile Nodes.

An example of the third approach is described in WO03036916, where a Mobile Node generates an authorisation certificate by signing a public key of the Mobile Router with its own private key. The certificate is then provided to the Mobile Router, which includes the certificate in any location update message it sends on behalf of the Mobile Node. The message is also signed with the private key of the Mobile Router. Using the public keys of the Mobile Router and the Mobile Node, a Peer Node can verify both that the update relates to the claimed Mobile Node and that the Mobile Router is authorised to perform the update on behalf of the Mobile Node. A Mobile Router may further delegate responsibility to another (upstream) Mobile Router by signing the public key of the upstream router with its own private key, adding this to the certificate, and passing the certificate to the upstream router. This approach necessitates however that the Peer Node process a relatively long certificate chain for every received location update.

SUMMARY

It is an object of the present invention to enable delegation of signalling rights in and between moving networks in such a way that the delegated rights can be passed between entities without introducing an excessive processing burden on Peer Nodes. This and other objects are achieved by providing a Mobile Router with a second symmetric key generated by a Mobile Node using a first symmetric key shared between the Mobile Node and a Peer Node. The Mobile Router is additionally provided with a “certificate” authenticating the second symmetric key using the first symmetric key. In this way, the mobile router can sign location update related messages sent to the Peer Node with the second symmetric key, and can provide the Peer Node with the certificate in order to allow the Peer Node to authenticate the right of the Mobile Router to act on behalf of the Mobile Node.

According to a first aspect of the present invention there is provided a Mobile Node arranged in use to communicate with a Mobile Router provisioning a wireless moving network, the Mobile Node comprising:

-   -   means for establishing a security association with a Peer Node,         said security association comprising a first symmetric key;     -   means for establishing a security association with said Mobile         Router;     -   means for delegating location update signalling rights for the         Mobile Node to the Mobile Router, the means for delegating being         arranged to provide the Mobile Router with an authorisation         ticket containing a second symmetric key derived from said first         symmetric key, and authorisation information identifying said         second symmetric key and confirming the authenticity of said         second symmetric key using said first symmetric key.

The various means recited above are typically implemented by way of a combination of software and data stored in a memory of the node, and hardware processing means, for example a microprocessor and RF circuitry.

The authorisation information provided to the Mobile Router is not bound to the Mobile Router. The information, or further information derived therefrom, can therefore be passed by the router to another upstream router or other upstream entity and can be freely used by the other router or entity.

Preferably, the Mobile Node is a Host Identity Protocol enabled node, and said means for establishing security associations with the Peer Node and with the Mobile Router are means for establishing Host Identity Protocol security associations. In this case, said first symmetric key is an HMAC key. Said second symmetric key is a further HMAC key derived from the first HMAC key, i.e. first symmetric key. Said authorisation information may comprise a lifetime of said second symmetric key, said second symmetric key, and an HMAC protecting the authorisation information and generated using said first symmetric key. The HMAC is protected with the end-to-end keying material.

Preferably, said authorisation information contains said second symmetric key integrity protected using said first symmetric key. The information may be encrypted using, for example, said first symmetric key. Rather than containing said second symmetric key, the information may contain a pointer to that key, for example in the case where the Mobile Node and the Peer Node share a common codebook, and said pointer is an index to the codebook.

According to a second aspect of the present invention there is provided a Mobile Router for provisioning a wireless moving network to Mobile Nodes, the Mobile Router comprising:

-   -   means for establishing a security association with a Mobile         Node;     -   means for receiving from the Mobile Node an authorisation ticket         containing a second symmetric key derived from a first symmetric         key known to the Mobile Node and to a Peer Node with which the         Mobile Node has established a security association, and         authorisation information identifying said second symmetric key         and confirming the authenticity of said second symmetric key         using said first symmetric key;     -   means for sending a location update message to said Peer Node on         behalf of the Mobile Node, the message containing a new location         of the Mobile Node and said authorisation information, and a         signature generated using said second symmetric key.

The various means recited above are typically implemented by way of a combination of software and data stored in a memory of the node, and hardware processing means, for example a microprocessor and RF circuitry.

The invention is applicable in particular to a Host Identity Protocol enabled router which is arranged in use to communicate with Host Identity Protocol enabled mobile and Peer Nodes. In this case, said first symmetric key is an HMAC key, and said second symmetric key is a further HMAC key.

Preferably, the Mobile Router comprises means for establishing a security association with a further Mobile Router or fixed network side node and for providing said ticket to that router or node, whereby the other router or node can perform location update signalling on behalf of the Mobile Node. Alternatively, the Mobile Router may derive a third symmetric key from said second symmetric key, and replace the second symmetric key in the ticket with the third symmetric key prior to sending the ticket to the further router or node. When sending a Location Update, the further Mobile Router or entity includes in the message the modified authorisation information, and signs the message with the third symmetric key.

According to a third aspect of the present invention there is provided a method of delegating location update signalling rights from a Mobile Node to a Mobile Router provisioning a wireless moving network, the method comprising:

-   -   establishing a security association between said Mobile Node and         a Peer Node, said security association comprising a first         symmetric key;     -   establishing a security association between said Mobile Node and         said Mobile Router;     -   sending an authorisation ticket from said Mobile Node to said         Mobile Router, the ticket containing a second symmetric key         derived from said first symmetric key, and authorisation         information identifying said second symmetric key and confirming         the authenticity of said second symmetric key using said first         symmetric key.

When it is necessary for the Mobile Router to send a location update to the Peer Node on behalf of the Mobile Node, the Mobile Router includes with the location update message said authentication information and a signature generated using said second symmetric key. Upon receipt of the message, the Peer Node identifies or obtains the second symmetric key, confirms the authenticity of the second symmetric key using the first symmetric key which is known to it, and authenticates the signature using the second symmetric key.

A preferred implementation of the method employs the Host Identity Protocol. In particular, said security associations are negotiated using the Host Identity Protocol, and said first and second symmetric keys are a first HMAC key and a second HMAC key derived from the first HMAC key. The location update message sent from the Mobile Router to the Peer Node is a Host Identity Protocol Location Update.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates schematically a moving wireless network comprising a Mobile Node attached to a Mobile Router;

FIG. 1B illustrates the network of FIG. 1A with the Mobile Node having been handed-off to a second Mobile Router;

FIG. 1C illustrates the network of FIG. 1A with the Mobile Node having being handed-off to a fixed access router;

FIG. 2 illustrates the HIP base exchange procedure;

FIG. 3 illustrates the HIP base exchange procedure with registration extension;

FIG. 4 illustrates an authentication ticket for inclusion in an 12 message of the HIP base exchange with registration extension;

FIG. 5 illustrates authentication information for inclusion in a location update;

FIG. 6 illustrates the signalling delegation process with sub-delegation from a first Mobile Router to a second Mobile Router; and

FIG. 7 illustrates use of a static signalling proxy in a fixed network with signalling delegation to the proxy.

DETAILED DESCRIPTION

The IETF document draft-ietf-hip-base-06 titled “Host Identity Protocol” introduces an addressing mechanism which disassociates a host's addressable identity (the “Host Identity”) from its physical location (i.e. a routable IP address). A Host Identity is in fact a public key of an asymmetric key pair owned by the host, which allows a host to prove ownership of a Host Identity whilst at the same time providing a means for establishing a secure communication channel between hosts. HIP requires the introduction into the protocol stack of a HIP layer which essentially performs a mapping between Host Identities (or Host Identity Tags HITs derived from Host Identities) and IP addresses. A given host can update its entry in the mapping table of a peer host by sending a Location Update to the peer. The sending host must sign a Location Update with a so-called HMAC key. This key (a “first” symmetric key) represents a shared secret between the hosts, and is determined during the HIP base exchange. The base exchange uses the Diffie-Hellman method as the basis for generating the HMAC key and the keys for the Encapsulating Security Payload (ESP) Security Association.

FIG. 2 illustrates the HIP base exchange carried out between a pair of HIP hosts, identified as a Mobile Node and a Peer Node. By way of example, the Mobile Node may be a mobile phone or the like, whilst the Peer Node may also be a mobile phone, or a fixed line user terminal or a network server. IETF draft “Host Identity Protocol (HIP) Registration Extension” (draft-ietf-hip-registration-02) describes an extension (introducing new TLVs) to the HIP base protocol intended to allow registration of a Mobile Node with a Mobile Router. The extended exchange is shown in FIG. 3. Registration related information is included in the exchanged messages. Once registered, the Mobile Node requests a routing service from the Mobile Router. It is proposed here to send a “delegation ticket” from the Mobile Node to the Mobile router as part of the registration extension, protected using the negotiated ESP Security Association. This ticket includes a new HMAC key (a “second” symmetric key) generated by the Mobile node using the keying material shared between the Mobile Node and the Peer Node. The ticket also includes the new HMAC key integrity protected and encrypted using the original HMAC key. As such, the Mobile Router cannot decrypt or modify this authentication information without this being apparent to any Peer Node (see below). As well as the integrity protected HMAC key, the authentication information contained within the ticket may include an authorisation lifetime and/or an authentication type (also integrity protected). FIG. 4 illustrates a posible implementation of the ticket, where the key K_(MN-PEER) is the original HMAC key known only to the Mobile Node and the Peer Node. K_(issuer-subject) is, in the first instance, a key shared between the Mobile Node (the key “issuer”) and the Mobile Router (the key “subject”). As the ticket is secured using the Security Association established between the Mobile Node and the Mobile Router, the new HMAC key is not leaked to any third party node.

For a Mobile Node newly registered with a Mobile Router, the Mobile Router must send a Location Update message to all Peer Nodes of the Mobile Node (where HIP associations have already been established between the Mobile Node and these Peer Nodes) identifying the new IP address of the Mobile Router. The Mobile Node will have notified the Mobile Router of the Host Identity Tag/IP address mappings during the base exchange, or these can be learned by the Mobile Router as packets pass through it in transit between the Mobile Node and the Peer Nodes. In the event that the Mobile Node is already attached to the Mobile Router when it attempts to connect to a new Peer Node, the base exchange with the Peer Node is run through and visible to the Mobile Router and the Mobile Router can establish the necessary state information. The Mobile Node also runs a registraton exchange with the Mobile Router to send the new ticket to the Mobile Router. In practice, the Mobile Node and the Mobile Router run the full registration exchange only once, and after establishing a security association between them, the Mobile Node can run an extended HIP update exchange with the Mobile Router. This enhanced three way update exchange includes the new authorization ticket for the Peer Node.

Similarly, when the Mobile Router changes its point of attachment (to the Internet), the Mobile Router must send Location Updates to all Peer Nodes associated with Mobile Nodes downstream of the Mobile Router. The Mobile Router signs each Location Update with the appropriate HMAC key for the Mobile Node and the Peer Node (nb. a Mobile Node generates a different HMAC key for each Peer Node). In addition, the

Mobile Router includes in the message the unmodified authentication information received from the Mobile Node for that Peer Node, i.e. the integrity protected (and encrypted) HMAC key. This information is illustrated in FIG. 5.

Rather than include the new HMAC key itself in the ticket, the Mobile Node may include only a pointer to such a new key. This may apply where the end-nodes, i.e. the Mobile Node and the Peer Node, generate initially shared keying material (HIP base exchange). The nodes have a lot of bulk keying material (random bytes in practise) that is stored as an array. The Mobile Node may send an index to the array that points to a specific part of the keying material. As a result, the end-nodes are kept in synchrony and they know which part of the keying material to use. In addition, the index, i.e. the pointer, does not reveal anything about the secret keys to outsiders.

The Mobile Router may construct the Location Update message such that it looks like an Update message that would have been sent by the Mobile Node, except for the inclusion of the authentication information and signature by the new HMAC key. In particular, the message may include the HIT of the Mobile Node as the source HIT. However, the message may also include a HIT of the Mobile Router as an additional parameter. This would allow, for example, the Peer Node to identify a different Security Association for use with this communication session. The message must also contain the index value for the HIT association and which is used by the Peer Node to identify the correct keying material, i.e. the original HMAC key. The index value is included in the authorisation ticket provided by the Mobile Node to the Mobile Router.

Upon receipt of a Location Update, a Peer Node must authenticate the right of the

Mobile Router to act on behalf of the Mobile Node. It does this by first verifying that the authentication information included with the Update is protected using the original HMAC key, and obtains that key either using a pointer contained in the update or directly (by decrypting the key if contained in the message). The Peer Node then knows whether or not the new HMAC key has been validly issued by the claimed Mobile Node (if the protected data includes only a pointer to a new HMAC key, the Peer Node must obtain or derive the new key). The Peer Node then checks whether or not the message is correctly signed with the new HMAC key, thus authenticating the Mobile Router. Assuming that the new HMAC key and Mobile Router are authenticated, the Peer Node updates its HIP mappings with the new location for the Mobile Node.

To avoid attacks related to the location update exchange, the Peer Nodes must send challenges to the Mobile Node's claimed location (i.e., reachability test). In practice, these challenge messages are destined to the Mobile Router. The Mobile Router on the forwarding path uses the new HMAC key to protect the reply message and sent the message back to the Peer Nodes on behalf of the Mobile Nodes.

It is important to note that no HIP base exchange is conducted between the Mobile Router and the Peer Node. No Security Association (SA) therefore exists between these nodes and no authentication process is conducted. The Peer Node trusts the Mobile Node to send the new HMAC key only to authorised Mobile Routers.

In the event that the Mobile Router moves behind another Mobile Router, the Routers must perform the HIP base exchange with registration extension. As a result, the upstream Router receives from the downstream Router authentication tickets for all Mobile Nodes that have delegated location update responsibility to the downstream Router. Each ticket contains the original authentication information provided to the downstream Mobile Router by the Mobile Node, and is signed by a key (K_(issuer-subject)) shared between the Mobile Routers. The upstream Router can then in turn perform location updates to Peer Nodes based on the information received in these tickets. As there is no association between the peer and the Mobile Router, there is no need to provide any additional information about the previous Mobile Router and the same authentication information can be used by the new Mobile Router for location updates. FIG. 6 illustrates the signalling flow associated with a Mobile Node (MN) first delegating signalling responsibility to a first Mobile Router (MR1), the first Mobile Router subsequently further delegating that responsibility to a second Mobile Router (MR2).

If a Mobile Node moves out of a mobile network, it must revoke the old HMAC key(s). It sends an update message to its Peer Node(s) identifying the keys that are no longer valid. Specifically, this may be done using an enhanced location update message which contains a hash of the revoked HMAC key. After receiving this information, a Peer Node does not accept any new location updates with the revoked key. Subsequently sent Location Updates are signed using the old HMAC key (or a further key if delegation to another Mobile Router occurs).

Considering now the nested moving network case, an extension to the HMAC key generation process makes it possible to revoke keys in the nested Mobile Router case. When a Mobile Router (#1) delegates signalling rights to another upstream Mobile Router (#2), the former does not reveal the initial location update HMAC key (provided by the Mobile Node). Instead, the Mobile Router #1 computes a one-way hash (e.g. SHA256) over the key received from the Mobile Node. The Mobile Router #1 replaces the (new) HMAC key within the ticket received from the Mobile Node, with the hashed key. The ticket still contains however the same authorisation information, namely the new HMAC key protected and encrypted with the original HMAC key. The ticket also includes an indication that the hashed key is the nth key in the hash chain. The modified ticket is sent securely to the second Mobile Router during the base exchange. The second, upstream Mobile Router uses the hashed key to protect its location update messages sent to the Peer Node, these messages also including the (original) authentication information. The Peer Node is able to authenticate the signature by itself decrypting the new HMAC key, and hashing that key (n times) to derive the signing key.

This approach makes it possible for the first Mobile Router to detach from the second Mobile Router and revoke the second Mobile Router's ticket. In other words, each Mobile Router in a nested moving network uses a hash value of a key used by the previous Mobile Router. The Mobile Routers lower in the hierarchy can revocate keys higher in the hierarchy.

Consider for example a key created as follows: MR#3-key=sha256(MR#2- key=sha256(MR#1-key)). Now, if the MR#2 leaves the MR#3 moving network, the former sends a revocation message to the Peer Node. The Peer Node knows that MR#2 key was higher in the key chain. Therefore, MR#2 is allowed to revoke the MR#3-key. As the Peer Nodes will always know the original key, they are able to compute all required hashed key values in the hash chain.

The procedures described above significantly reduce location update related signalling as a Mobile Router acts as a signalling proxy between Mobile Nodes and a Peer Nodes, performing location updates en bloc for all downstream Mobile Node. Moreover, the procedures are simpler to implement and operate than the prior art public-private key pair based certificate approach, as authentication by a Peer Node requires only the authentication of a pair of symmetric key signatures (performed using the old and the new HMAC keys).

In order to optimise the over-the-air signalling between the Mobile Routers and the Internet, a static signalling proxy may be introduced as illustrated in FIG. 7. The

Mobile Router registers its clients and delegates the signalling rights to the static signalling proxy located in the fixed network. When the Mobile Router makes a hand- off, it runs a single location update exchange with the static signalling proxy. The Mobile Router's multiplexed address (i.e. the combination of the Router's IP address and some identity such as port number or HIT) may represent the current location of all the clients belonging to the moving network behind it. The static signalling proxy in the fixed network can use the available high bandwidth to send a “burst” of location updates to the Peer Nodes on behalf of the Mobile Nodes. The Peer Nodes send the challenge messages to the multiplexed locator of the Mobile Router if the static signalling proxy is not on the forwarding path in order to verify that the Mobile Nodes are at the location where the static signalling proxy claims them to be. If the static signalling proxy is on the forwarding path, the proxy replies directly to the challenge messages sent by the Peer Nodes.

The procedures described here have some similarities with the prior art Kerberos model for establishing authenticated security associations. Kerberos is described in IETF RFC4120. In particular, the Mobile Node acts like a Key Distribution Center (KDC). However, according to Kerberos, the ticket provided to a client is bound to that client. The Kerberos approach therefore precludes, in a scaleable way, further delegation to an upstream router. In contrast, the approach described here does not bind the ticket to any particular entity and as such can be transferred upstream without approval from the originating Mobile Node.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. 

1. A Mobile Node arranged in use to communicate with a Mobile Router provisioning a wireless moving network, the Mobile Node comprising: means for establishing a security association with a Peer Node, said security association comprising a first symmetric key; means for establishing a security association with said Mobile Router; means for delegating location update signalling rights for the Mobile Node to the Mobile Router, the means for delegating being arranged to provide the Mobile Router with an authorisation ticket containing a second symmetric key derived from said first symmetric key, and authorisation information identifying said second symmetric key and confirming the authenticity of said second symmetric key using said first symmetric key.
 2. A Mobile Node according to claim 1, wherein the Mobile Node is a Host Identity Protocol enabled node, and said means for establishing security associations with the Peer Node and with the Mobile Router is arranged to establish Host Identity Protocol security associations.
 3. A Mobile Node according to claim 2, said first symmetric key being an HMAC key.
 4. A Mobile Node according to claim 3, said second symmetric key being a further HMAC key derived from said first mentioned HMAC key.
 5. A Mobile Node according to claim 1, said authorisation information comprising a lifetime of said second symmetric key, said second symmetric key, and a signature protecting the authorisation information and generated using said first symmetric key.
 6. A Mobile Node according to claim 1, said authorisation information containing said second symmetric key integrity protected using said first symmetric key.
 7. A Mobile Node according to claim 1, said authorisation information containing a pointer to said second symmetric key.
 8. A Mobile Node according to claim 1, said authorisation information being encrypted using said first symmetric key.
 9. A Mobile Router for provisioning a wireless moving network to Mobile Nodes, the Mobile Router comprising: means for establishing a security association with a Mobile Node; means for receiving from the Mobile Node an authorisation ticket containing a second symmetric key derived from a first symmetric key known to the Mobile Node and to a Peer Node with which the Mobile Node has established a security association, and authorisation information identifying said second symmetric key and confirming the authenticity of said second symmetric key using said first symmetric key; means for sending a location update message to said Peer Node on behalf of the Mobile Node, the message containing a new location of the Mobile Node and said authorisation information, and a signature generated using said second symmetric key.
 10. A Mobile Router according to claim 9, the Router being a Host Identity Protocol enabled router which is arranged in use to communicate with Host Identity Protocol enabled mobile and Peer Nodes.
 11. A Mobile Router according to claim 10, wherein said first symmetric key is an HMAC key, and said second symmetric key is a further HMAC key.
 12. A Mobile Router according to claim 9 and comprising means for establishing a security association with a further Mobile Router or fixed network side node and for providing said ticket to that router or node, whereby the other router or node can perform location update signalling on behalf of the Mobile Node.
 13. A Mobile Router according to claim 12 and comprising means for deriving a third symmetric key from said second symmetric key, and replacing the second symmetric key in the ticket with the third symmetric key prior to sending the ticket to the further router or node.
 14. A method of delegating location update signalling rights from a Mobile Node to a Mobile Router provisioning a wireless moving network, the method comprising: establishing a security association between said Mobile Node and a Peer Node, said security association comprising a first symmetric key; establishing a security association between said Mobile Node and said Mobile Router; sending an authorisation ticket from said Mobile Node to said Mobile Router, the ticket containing a second symmetric key derived from said first symmetric key, and authorisation information identifying said second symmetric key and confirming the authenticity of said second symmetric key using said first symmetric key.
 15. A method according to claim 14, wherein said security associations are established using the Host Identity Protocol.
 16. A method of performing a location update in respect of a Mobile Node following delegation of location update signalling rights from the Mobile Node to a Mobile Router according to the method of claim 14, the method comprising including in a location update message sent from the Mobile Router to a Peer Node, said authentication information and a signature generated using said second symmetric key, and, upon receipt of the message at the Peer Node, identifying or obtaining the second symmetric key, confirming the authenticity of the second symmetric key using the first symmetric key, and authenticating the signature using the second symmetric key.
 17. A method according to claim 16, said location update message being a Host Identity Protocol Location Update. 